Autosave is always working and cannot be turned on or off. It does not save to your existing project. It saves to a hidden file in your app data directory, and then that is checked when you reopen a project. It is only meant for crash recovery. Every 30 seconds it checks if the project is modified (window title bard shows an asterisk except macOS which uses the red dot) and saves to this backup.
In case you think it is not working, make a change, wait 30 seconds, and look at the end of View > Application Log for [Debug ] <autosaveTask> Function autosaveTask finished. It also shows above this the path and name of the hidden backup file, but once you save successfully including save at exit when prompted, the backup file is removed.